Bisphenol-A has been an extremely useful chemical for many decades. AS a difunctional monomer, it has been used in the preparation of numerous polymers. For example Bisphenol-A [2,2'-bis(4-hydroxyphenyl)propane] has been utilized in preparing such materials as epoxy resins, polyetherimides, polyarylates and, in particular, polycarbonates. In certain of these polymer systems, particularly the epoxy systems, the purity of the Bisphenol-A (hereinafter referred to as BPA) employed in the polymer reaction need not be that high. Epoxy resins only need BPA of approximately 95% purity. The impurity which is present in the greatest amount in such systems is generally orthopara BPA. However with other polymer systems, particularly polycarbonates, the purity of the BPA must be substantially higher. Purities of BPA of about 99.50% or higher preferably 99.80 or 99.90% or higher are desirable and in many cases necessary for the preparation of BPA polycarbonates. Therefore there has been substantial attention directed to the preparation and purification of BPA.
The art is replete with references directed to the preparation of BPA. Usually this is done by the condensation of phenol with acetone in the presence of a catalyst system. Generally the catalyst is an acidic catalyst. For many years one of the particularly useful catalyst systems in the patent art and employed commercially was hydrochloric acid. Although the economics of the process are initially good with respect to the conversion of the reactants to BPA, the maintenance of the apparatus is costly. The hydrochloric acid is extremely corrosive and ordinary metallic reactors and piping must be changed on a frequent basis. Obviously glass lined reactors or certain alloyed metals can be employed, however, these are quite expensive. In later years there seems to be the tendency to use heterogeneous acidic catalyst system wherein the acidic catalyzation occurs at the catalyst surface and is actually bound to the catalyst. in this manner the "acid" does not flow with the unused reactants and BPA. Such catalyst systems are generally sulfonated polystyrenes which are substantially crosslinked such as the Amberlites and like materials. However, such sulfonated polystrenes, because they contain sulfonic pendent groups may form sulfonic acids. The sulfonic acids, along with hydrochloric acid, if employed as part of or a contaminant in the catalyst system, and any other acids can be very corrosive to the equipment, particularly rotary vacuum pumps. Consequently, aqueous caustic has been used as the liquid vacuum seal in rotary vacuum pumps in order to neutralize the acids in the BPA condensation reaction products generally comprised of BPA, phenol, and water and other materials. The reaction products are sent to a crystallizer system wherein a BPA/phenol crystal slurry is formed. The slurry is sent to a rotary vacuum filter, equipped with vacuum pumps which pull the slurry through a filter drum. A cake (BPA/phenol adduct) is formed on the drum and the extracted liquid often referred to as the mother liquor is recycled to the reactor in which BPA is formed. The mother liquor may be subjected to dehydration in order to remove water of reaction and other unwanted materials before recycling the mother liquor back to the reactor. Optionally, the BPA adduct may be redissolved in phenol and sent to a second rotary filter (second stage) equipped with vacuum pull for further purification.
After the BPA is prepared, various isolation and purification procedures are known. Many of these appear in the relatively voluminous patent art. Generally phenol is distilled off to a great extent and/or the initial purification by adduct crystallization of the BPA/phenol adduct Distillation of BPA itself can also be employed. The purification of the BPA can then be further accomplished through the addition of various organic solvents such as toluene or methylene chloride so as to remove the BPA from various impurities. Additionally water and various glycols such as ethylene glycol and glycerin have been used alone or together to separate and thus purify the BPA from its impurities.
Therefore, it is an object of this invention to provide a process whereby sodium ions are not introduced into the BPA.
Another object of this invention is to employ liquid phenol as a seal liquor in rotary vacuum pumps.
The foregoing and other objects of this invention will become apparent from the following description and appended claims.